1. Field of the Invention
The present invention relates to an improvement in stencil printing, and more particularly, to control of the transfer of ink through perforations of a stencil sheet.
2. Description of the Prior Art
The stencil printing comprises the steps of supplying ink in the form of a layer on a first surface of a perforated stencil sheet, and applying a pressure to the ink layer so as to transfer the ink through perforations of the stencil sheet from a first side thereof facing said first surface to a second side thereof facing a second surface opposite to said first surface and to attach the ink thus transferred through the perforations of the stencil sheet onto a surface for printing.
A great difference in stencil printing from other printing arts such as anastatic printing, intaglio printing or offset printing is in that the ink is continuously supplied with no distinct division for each of a number of printings (or a number of print sheets) produced in succession. In other words, in stencil printing, the amount of ink supplied on one surface of a perforated stencil sheet in the form of a layer is not so small as to be exhausted by one time printing (or one sheet printing) but generally is adequate so as to be enough to print several times (or several print sheets). Particularly in stencil printing using a less fluidal ink, getting more popular these years, the ink layer supplied on one surface of a perforated stencil sheet is often so thick as to afford more than ten or sometimes hundreds of prints without replenishment of the supply of ink in the meantime.
Thus, stencil printing is carried out by supplying an ink layer having a capacity of printing tens to hundreds of prints on a first surface of a perforated stencil sheet, contacting a second surface opposite to said first surface of the stencil sheet with a surface for printing, applying a pressure to the ink layer by a pressing means so as to transfer the ink of the ink layer through perforations of the stencil sheet from a first side thereof facing said first surface toward a second side thereof facing said second surface, attaching the ink thus transferred onto the surface for printing, then releasing the pressure applied to the ink layer, and then detaching the surface for printing from the stencil sheet. In this case, the matter of how much ink is transferred from the ink layer onto the surface for printing as separated from the ink layer when the surface for printing is detached from the stencil sheet depends on the fluidity and the adhessiveness of the ink, the affinity between the ink and the surface for printing, the size of the perforations, etc. If the amount of transfer of the ink is too much, a blurring of the printed image occurs, while if the amount of transfer of the ink is too small, the printed image becomes dim.
When the stencil sheet is perforated by a thermal copying method according to which a thermo-sensitive stencil sheet including a thermoplastic film is laid over an original bearing black images containing carbon or the like which generates heat by absorbing infrared rays, and light beams rich in infrared rays are radiated onto the black images through the thermoplastic film, so that portions of the thermoplastic film corresponding to the black images are melted by the heat generated in the black image and form perforations following the black images of the original, the size of each perforation differs widely according to the size of the black images. Therefore, it is very difficult to optimize the fluidity and the adhessiveness of the ink to match the wide variety of the size of the perforation so that the amount of transfer of the ink through the perforation is placed under the control of the flow resistance applied to the throttling action by the perforation. Therefore, if the printed image is not be dim at a small perforation, the printed image would unavoidably show a tendency of blurring at a large perforation due to an excessive transfer of ink.
When such a thermo-sensitive stencil sheet is perforated by a dot matrix thermal perforation using a dot thermal head, each perforation formed in the stencil sheet generally has a substantially uniform size regardless of the size of the black portion of the image. However, even in this case, since the amount of transfer of the ink from the ink layer onto the surface for printing at each dot printing is determined according to the cutting condition of the ink from the continuous ink layer, if no dim portion is to occur at any portion of the entire image, it is unavoidable that the amount of transfer of the ink is generally adjusted to be larger than that in anastatic, intaglio or offset printing. Therefore, even in this case, when the printed sheets are stacked immediately after the printing, there is still the problem of the back contamination, although it is not so serious as in the case of the thermal copying stencil printing.